Article 48130 of sci.electronics:
Newsgroups: sci.electronics
Path: bcars267!mwandel
From: mwandel@bnr.ca (Markus Wandel)
Subject: Re: Circuit to use old phones as an inter-comm needed
Message-ID: <1993Jul28.124912.24689@bnr.ca>
Sender: news@bnr.ca (usenet)
Nntp-Posting-Host: bcars758
Organization: Bell-Northern Research, Ottawa, Canada
References: <16C19D768.A8637710@tscc.macarthur.uws.edu.au>
Date: Wed, 28 Jul 1993 12:49:12 GMT

In article <16C19D768.A8637710@tscc.macarthur.uws.edu.au> A8637710@tscc.macarthur.uws.edu.au writes:
>I have managed to get a hold of some old pulse dialing phones. I thought it
>would be a good idea to set them up as an Inter-comm so the kids can use them.
>Is there any netter out there that may out me on the right track to do this.

I have recently thought about this and come up with a kludgy but workable
scheme.

Talking over the phones is easy.  You put DC current through the phone and it
transmits and receives audio.  So two phones and a current source (about 25mA)
all in series will give you a talking circuit.  A suitable current source can
be as simple as a 9V battery and a series resistor whose value is adjusted
(with both phones offhook) till about 25mA flows.  You can then bypass the
battery and the resistor with a capacitor to couple the audio straight across
and get a loud and clear connection.

What is much harder is signaling the other end.  To ring the bell you need to
put 90V (RMS) 20Hz AC into the phone (nominally).  Lower voltages will work
(down to about 40V) but different frequencies won't.  You can't ring the 
phone at 60Hz.  I have a ringing circuit in a PBX I built but it consists of
a 20Hz sinewave generator, a push-pull power booster and a big transformer.
Much too elaborate for a simple 2-phone intercom circuit, and anyway the
ringing voltage could painfully zap a kid.

So forget the bell and look into other forms of signaling.  This is what I have
come up with:

                              +  | | -
      +-------+------ - - --+---||||---/\/\/--+---- - -----+-------+
      |       |             |    | |     R    |            |       |
      |       |             | 24V             |            |       |
      |      ---            |                 |           ---      |
      |     |   |           +---||------------+          |   |     |
      |      --- Sonalert       C                Sonalert ---      |
      |   C   |                                            |   C   |
      +---||--+                                            +--||---+
      |      _|_,                                         _|_      |
      |      / \  15V                               15V   \ /      |
    PHONE    -+- Zener                             Zener `-+-    PHONE
      |       |                                            |       |
      |       |                                            |       |
      +-------+------------------ - - - -------------------+-------+

As before, set R to give you a talking current (both phones offhook) of about
25mA.  Start with 1K ohm.  Leave it in if the phones work well enough; the
current is not very critical.  The capacitors C are audio bypass capacitors
and should be about 0.47uF.

When the phones are onhook they present an open circuit, and the 24V battery
voltage is not enough to overcome the 30V series drop of the Zeners and no
current flows.  When both phones are offhook they present a very low resistance
and the talking current (determined by R) flows.

When only one phone is offhook it places its low DC resistance across the Zener
diode on its side so that the full 24V supply is applied to the other side.
This overcomes the voltage drop of the other Zener diode so the other Sonalert
beeps.  The wonderful thing about Sonalerts is that they make a loud noise with
only a few milliamps of current so the series resistor R doesn't matter.
Especially nice is a pulsing Sonalert which goes "Beep beep beep"
automatically.  While the far-end Sonalert is beeping, you hear the beeping
in the near-end receiver (at low volume thanks to the bypass capacitor across
the far-end Sonalert) to confirm that the line is working and the other end is
being signaled.

The power supply can be three 9V batteries in series but since 80% of the power
is lost in series resistor R rather than in powering the phones it seems a
little wasteful.  A 24V wall wart with clean filtering would be better.

The signaling components can be mounted inside the phones.  Only two wires are
needed to go to each phone, and the power supply can be mounted centrally,
out of harm's way.  If R is adequately big (1/2 watt) and has enough
ventilation then both lines can be indefinitely shorted out without any fire
hazard and there is not enough voltage anywhere to hurt anyone.

I have tested this with 500-type phones and two different types of piezo
buzzers (pulsing sonalerts and non-pulsing brand X ones) and it works great.
You should be able to get all the needed parts including piezo buzzers at
Radio Shack.  I love telephones.  Too bad I don't have any kids who want an
intercom line.
-- 
Markus Wandel     Ottawa Ont. Canada   (613) 592-1225
markus@pinetree.org  <-- NOT 'mwandel@bnr.ca', that does not work.